Zeni Farming 1

Once you unlocked the Parallel Quest: "Prepare for the Attack of Saiyans", defeat Krillin, Yamcha, and Tien in the first part of the battle.

Once the gate to the next area is unlocked, don't head there first. Look out for a random Time Patroller that will appear in one of the building's entrances. If he's not there, Retry from the beginning, defeat the three opponents again, then look for him. Keep doing this until he appears. His appearance will change from time to time since this is randomly generated. Talk to him and accept his challenge. Deal enough damage to him until you get a notification that you got a Material item.

Now proceed and complete the rest of the quest. At the evaluation screen, you should get a Hercule Badge.

Back in the city, you can now sell the Hercule Badges you collected for a good amount of Zeni!